In detecting impedance rheogram (plethysmogram), a small and high-frequency constant current is applied to a part or an organ of a subject's body, and at the same time, the variation of the electric impedance of the part or organ caused by the change of blood volume in a blood vessel during a heart beat process is detected and recorded, and the recorded waveforms are called the impedance rheogram. In practice, the rheograms detected at different part or organs are respectively called rheocardiogram, pulmonary rheogram, rheohepatogram, rheoencephalogram, and the like. The present invention relates to the former three kinds of rheogram which are apt to be interfered with by a subject's respiration.
When the impedance rheograms of heart, lung and liver are detected by conventional means, the variation of impedance will also be caused by the change of air volume in the lungs during respiration, thus the detected results of the impedance rheogram will be seriously distorted. In order to avoid respiration interference, the subjects are required to hold their respiration during detection, but this requirement is almost impossible for those having serious heart problems, for those who are respiration problems, unconscious or for young babies. Therefore, the conventional apparatus and methods can not be used on these subjects.
Besides the respiration interference, the detection of the impedance rheogram is also apt to be interfered with by many internal factors (such as subject's body movements electromyographic and electrocardiographic signals, etc) as well as external factors (such as environmental interference and instrument noise). All of these factors will reduce the reliability of the detection results.
In an article entitled "Microprocessor Impedance Rheogram System" by Liu Ying, in APPLICATION OF ELECTRONIC TECHNOLOGY, volume 1, 1984, PP 17-19, there is disclosed a system for eliminating respiration interference of the differential impedance rheocardiogram dz/dt by way of digital superposing. However, the superposing thereof is done according to a constant sampling frequency, which is neither adequate for those having arrhythmia nor for those having heart rate change during detection. It has been proved by clinical data that many healthy people's heart rate is changed under free respiration, and serious arrhythmia may be found in many patients having heart problems, and all these will certainly change the cyclic waveforms of the impedance rheogram. Under such situations, the superposition processing on the basis of sampling with a constant time interval will cause significant errors, thus rendering it useless in clinical practice.